Ukraine launched Thursday construction of a new shelter to permanently secure the stricken Chernobyl plant as it marked the 26th anniversary of the world's worst nuclear disaster.

President Viktor Yanukovych pressed a symbolic button at the construction site, watched by workers and ambassadors from countries including China and Japan that contributed to the huge project, expected to cost 1.5 billion euros.

"In the name of Ukraine, I express my deep thanks to all the donor countries to the Chernobyl Shelter Fund for their understanding and effective aid to our country in overcoming the consequences of the worst man-made disaster in human history," Yanukovych said, as cranes loomed over the site.

"We have felt that the whole world has come to help us."

An explosion during testing at the power plant in the early hours of April 26, 1986, sent radioactive fallout into the atmosphere that spread across Europe, particularly contaminating Belarus, Ukraine and Russia.

An international drive has raised funds from governments towards building a new permanent covering to slide over a temporary concrete-and-steel shelter that was hastily erected after the disaster and has since developed cracks.

The 20,000-tonne arched structure that spans 257 metres, known as the New Safe Confinement, is designed to last for a century, and will contain hi-tech equipment to carry out safe decontamination work inside the ruined reactor.

"This construction in its scale has no equals in the world," Yanukovych said.

The construction of the shelter is expected to cost 990 million euros and to be put in place in 2015, while the decontamination work on the site will push the total cost up to 1.5 billion euros ($2 billion).

Yanukovych said in his speech at the site that "the completion of this project will guarantee the environmental safety of all the surrounding land and make it impossible for radiation to reach the atmosphere."

Chernobyl is only around 100 kilometres (60 miles) from Kiev and lies close to the borders with Russia and Belarus. The area around the plant is still very contaminated and is designated as a depopulated "exclusion zone."

International donors have so far agreed to contribute 550 million euros ($730 million) to the project, with the balance coming from the European Bank of Reconstruction and Development.

As the president visited the site, some 1,000 Chernobyl clean-up workers rallied in Kiev over cuts to their benefits in the latest of a string of angry protests over the austerity measure, the Interfax news agency reported.

The Soviet Union ordered thousands of people to take part in the clean-up in Ukraine following the Chernobyl accident, working without adequate protection.

Although only two people were killed in the initial explosions, the United Nations atomic agency says that 28 rescue workers died of radiation sickness in the first three months after the accident.

According to Ukrainian official figures, more than 25,000 of the cleanup workers, known as "liquidators" from then-Soviet Ukraine, Russia and Belarus have died since the disaster.

Ukraine on Thursday announced that it was awarding state honours to more than 40 of the liquidators and Yanukovych laid flowers and held a minute's silence at a memorial at the power station.

On Thursday, relatives of victims held a remembrance ceremony in front of a memorial in Kiev as soldiers in dress uniform stood guard. Chernobyl veterans also attended a memorial ceremony in Minsk.

In neighbouring Belarus which also suffered from Chernobyl's nuclear fallout about 2,000 supporters of the former Soviet republic's opposition took to the streets to commemorate the disaster and denounce political repression.

Demonstrators brandished banners saying "We are suffering from Chernobyl", "There's no such thing as civilian nuclear energy", and "No to nuclear power plants in Belarus".

Other demonstrators shouted anti-regime slogans such as "Freedom to political prisoners".

photo-zzh-am/ma/gk/db